Titanium based composites and coatings and methods of production

ABSTRACT

A titanium based composite which includes a Ti(Al,O) base matrix, discrete ceramic particles, and an oxide layer on the surface of the composite. The discrete ceramic particles are integrally associated with the Ti(Al,O) base matrix and the oxide layer, so that at a temperature of above about 600° C., the composite is substantially resistant to oxidation and spallation.

TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to the use of titanium based composites foruse in component manufacture, or for use as coatings, with improvedresistance to high-temperature oxidation and spallation. The inventionalso relates to titanium-based composites and to methods of production.

BACKGROUND TO THE INVENTION

[0002] Titanium based alloys and intermetallic compounds are, ingeneral, known. For example, titanium aluminide (Ti₃Al) basedintermetallic compounds are attractive structural materials forapplications in the aerospace and automobile industries because of theirlow density, high melting point, and high specific strength andexcellent mechanical properties. However, the industrial application ofsuch materials has been limited.

[0003] Ti₃Al based intermetallic compounds show a relatively lowductility and poor high-temperature oxidation resistance. Temperaturesover approximately 650° C. cause cracking to the oxide layer of theseTi₃Al based compounds. This leads to spallation of the outer oxide layerof the compounds and rapid oxidation of the underlying material.Continued exposure of the Ti₃Al based compounds to such environmentseventually leads to the degradation and destruction of the materials.For these reasons, Ti based alloys and intermetallic compounds have beenrestricted in application to temperatures below 650° C., as at aboutthis temperature the materials become oxidised quickly.

[0004] Al₃Ti coating, obtained using conventional pack cementation, canimprove the oxidation resistance of Tl₃Al, because the outer oxide layeror scale formed is composed mainly of α-Al₂O₃. However, such coatingsare not ideal as they form small but permeable cracks, which penetratethe coating layers and compromise the oxidation resistance. As a resultthe materials have limited applications.

[0005] To improve the mechanical and oxidation properties, Ti₃Al hasbeen alloyed with Nb, Cr, Mo, Si and/or W and this has shown somebenefits. The main problem with alloying methods is that one specialelement cannot improve all required properties to a desirable level.Multi-element alloying is therefore often used, and microstructuralcontrol with thermal or thermo-mechanical treatment is required in mostcases. Such methods are complicated and expensive.

[0006] It will be appreciated by those skilled in the art that iftitanium based materials are to have a wider range of commercialapplications at high temperatures, they must be substantially resistantto oxidation and spallation at high temperatures; easy to prepare andfabricate; and be cost efficient.

OBJECT OF THE INVENTION

[0007] With the above background in mind, it is an object of thisinvention to provide titanium based composite materials which address orat least ameliorate disadvantages of known titanium based alloys andintermetallic compounds, or at least which will provide the public witha useful alternative.

[0008] Further objects of this invention will become apparent from thefollowing description which is given by examples only.

SUMMARY OF THE INV NTI N

[0009] According to one aspect of this invention there is provided atitanium based composite which includes a Ti(Al,O) base matrix, discreteceramic particles, and an oxide layer on the surface of the composite,wherein the discrete ceramic particles are integrally associated withthe Ti(Al,O) base matrix and the oxide layer, and wherein, at atemperature of above about 600° C., the composite is substantiallyresistant to oxidation and/or spallation.

[0010] Preferably the discrete ceramic particles range in size from 0.1μm to 30 μm.

[0011] Preferably the discrete ceramic particles are selected fromAl₂O₃, SiC, TiC, TiN, TiB₂, Y₂O₃ and/or Si₃N₄.

[0012] Preferably the discrete ceramic particles may constitute a volumefraction of about 10% to 60% of the titanium based composite.

[0013] According to a further aspect of this invention there is provideda coating material including titanium based composite adapted for use onsubstrate components used at high temperature and/or in oxidativeenvironments, wherein the composite includes a Ti(Al,O) base matrix,discrete ceramic particles and an oxide layer, wherein the discreteceramic particles are integrally associated with the Ti(Al,O) basematrix and the oxide layer so that at a temperature of above about 600°C. the composite is substantially resistant to oxidation and/orspallation.

[0014] Preferably the discrete ceramic particles range in size from 0.1μm to 30 μm.

[0015] In one preferred form the discrete ceramic particles are selectedfrom Al₂O₃, TiC, SiC, TiN or TiB₂.

[0016] Preferably the ceramic particles constitute a volume fraction ofabout 10% to 60% of the titanium based composite.

[0017] Preferably the composite is resistant to oxidation and/orspallation at temperatures between 600° C. and 900° C. and morepreferably above 700° C.

[0018] According to a further aspect of this invention there is provideda method of producing a coating for application to a component used attemperatures above 600° C. and/or in oxidative environments, wherein themethod includes the steps of:

[0019] preparing a Ti(Al,O) based composite powder, with each of thepowder particles including discrete Al₂O₃ particles, according to themechanical milling and thermal treatment method disclosed inPCT/NZ98/00124;

[0020] applying the composite powder produced to a substrate componentto produce a composite coating; and

[0021] exposing the coated component to a high temperature, oxidativeenvironment above about 600° C. to form a surface oxide layer on thecomposite coating.

[0022] Preferably the composite powder is applied to the substrate usinga thermal or plasma spray process.

[0023] Preferably the coated component is heated to between about 700°C. and about 900° C. for between about 1 and 200 hours in an oxygencontaining environment to form the surface oxide layer.

[0024] Preferably the coated component is heated in an oven before useor is heated in situ during use.

[0025] According to a further aspect of the invention, there is provideda process for producing a titanium based composite material in apre-selected form including the steps of:

[0026] preparing a Ti(Al,O) based composite powder with each of thepowder particles, including discrete Al₂O₃ particles, according to themechanical milling and thermal treatment method disclosed inPCT/NZ98/00124;

[0027] pressing the powder formed into a pre-selected mould to produce apowder compact and sintering the powder compact at a temperature ofabove about 700° C. under an inert environment;

[0028] exposing the sintered composite material or component to a hightemperature, oxidative environment above about 700° C. to form a surfaceoxide layer;

[0029] wherein the product produced is substantially resistant tooxidation and/or spallation at temperatures above 600° C.

[0030] Preferably the sintering temperature is between 700° C. and 1650°C.

[0031] Preferably the inert environment is a vacuum or argonenvironment.

[0032] According to a further aspect of this invention there is provideda method of producing a coating for application to a component used attemperatures above 600° C. and/or in oxidative environments, wherein themethod includes the steps of:

[0033] preparing a Ti(Al,O) based composite powder, with each of thepowder particles including discrete TiC, SiC, TiN, TiB₂, Y₂O₃ and/orSi₃N₄ particles, according to the mechanical milling method disclosed inPCT/NZ98/00124;

[0034] applying the composite powder produced to a substrate componentto produce a composite coating; and

[0035] exposing the coated component to a high temperature, oxidativeenvironment above about 600° C. to form a surface oxide layer on thecomposite coating.

[0036] Preferably the composite powder is applied to the substrate usinga thermal or plasma spray process.

[0037] Preferably the coated component is heated to between about 700°C. and about 900° C. for between about 1 and 200 hours in an oxygencontaining environment to form the surface oxide layer.

[0038] Preferably the coated component is heated in an oven before useor is heated in situ during use.

[0039] Preferably the component is to be used at temperatures between600° C. and 900° C.

[0040] According to a further aspect of the invention, there is provideda process for producing a titanium based composite material in apre-selected form including the steps of:

[0041] preparing a Ti(Al,O) based composite powder, with each of thepowder particles including discrete TiC, SiC, TiN, TiB₂, Y₂O₃ and/orSi₃N₄ particles, according to the mechanical milling method disclosed inPCT/NZ98/00124;

[0042] pressing the powder formed into a pre-selected mould to produce apowder compact and sintering the powder compact at a temperature ofabove about 700° C. under an inert environment;

[0043] exposing the sintered composite material or component to a hightemperature, oxidative environment above about 700° C. to form a surfaceoxide layer;

[0044] wherein the product produced is substantially resistant tooxidation and/or spallation at temperatures above 600° C., preferablybetween 600° C. and 900° C.

[0045] Preferably the sintering temperature is between 700° C. and 1650°C.

[0046] Preferably the inert environment is a vacuum or argonenvironment.

[0047] Other aspects of the invention will become clear from thedisclosure below which is given by way of example only with reference tothe figures.

DESCRIPTION OF FIGURES

[0048] The present invention will be described with reference to theaccompanying figures in which:

[0049]FIG. 1 shows a SEM micrograph of the Ti(Al,O)/Al₂O₃ compositeproduced by sintering the Al/TiO₂ composite powder at 1550° C. for 1hour. The dark particles are Al₂O₃ particles.

[0050]FIG. 2 shows SEM micrographs of the Ti(Al,O)/Al₂O₃ compositeproduced by pressureless sintering of the Al/TiO₂ composite powder at(a) 1550° C. and (b) 1650° C. for 1 hour respectively.

[0051]FIG. 3 shows SEM backscattered electron micrographs of theTi(Al,O)/SiC composite which include 10 vol. % SiC ceramic particles.The composite is produced by HiPping at 1000° C. for 2 hours under 200MPa: (a) 2 h milled and (b) 8 h milled.

[0052]FIG. 4 shows surface and cross-section morphologies ofTi(Al,O)/Al₂O₃ composite after oxidation at 700° C. isothermally for 100hours: (a) surface morphology; (b) and (c) cross-section morphology.

[0053]FIG. 5 shows a cross-section of Ti(Al,O)/TiC composite with 20vol. % of TiC oxidised at 800° C. in air for 200 hours; (a) producedusing 8 hrs milled powder, and (b) produced using 16 hrs milled powder.

[0054]FIG. 6 shows SEM micrographs of the Ti(Al,O)/SiC with 10 vol. % ofSiC powder particles after different milling durations (a) 2 hours; (b)4 hours; (c) 8 hours; and (d) 16 hours.

DETAILED DESCRIPTION OF THE INVENTION

[0055] The formation of composites is a relatively new approach to theuse of titanium based materials. The present invention is directed atthe selection and use of high temperature and oxidation resistanttitanium based composites, for producing components of use in industriessuch as the aerospace and automobile industries.

[0056] Applications for titanium based composites can include coatingsfor engine components, such as compressor discs and blade rings, whichare used in high temperature and highly oxidative environments. Suchuses require a satisfactory level of resistance to oxidation and damageof the coating and the underlying substrate component. The compositescan also be used to form the component itself. The present invention mayalso be seen to relate to a process for preparing such composites andthe application of them to engineering components.

[0057] The titanium based composites of the present invention include atitanium base matrix referred to herein by the general formula Ti(Al,O);discrete ceramic particles; and an oxide layer on the surface of thecomposite.

[0058] The Ti(Al,O) matrix is primarily composed of titanium, aluminiumand oxygen. Preferably the titanium matrix includes about 0.1 at. % toabout 30 at. % dissolved oxygen and about 15 at. % to about 30 at. %aluminium.

[0059] The discrete ceramic particles are selected from Al₂O₃, SiC orTiC. Other ceramic particles of use will include TiN and TiB₂. Otheralternatives, such as Y₂O₃ and Si₃N₄, known to the skilled person canalso be used.

[0060] When the discrete ceramic particles are Al₂O₃, the composite ispreferably produced using the process as disclosed in PCT/NZ98/00124 andits corresponding U.S. Pat. No. 6,264,719 (the disclosure of which isincorporated herein by way of reference). The process first produces aTiO₂/Al composite powder by high energy mechanical milling. Thiscomposite powder consists of fine fragments including a mixture of finephases mainly of TiO₂ and Al, having a particle size of less than about500 nm. Then the TiO₂/Al composite powder is thermally treated tofacilitate the reaction between TiO₂ and Al, forming Ti(Al,O)/Al₂O₃composite powder. For the other ceramic particles (SiC, TiC etc) thisthermal treatment step is omitted. Using SiC as an example, the powderwill be produced by the mechanical milling of SiC powder, Ti powder, andAl powder, followed by the same later steps as for Al₂O₃. Proportionsused would be determined by the composition of the final material asdesired. The volume fraction of the discrete ceramic particles in thepowder may vary in range of about 10% to 60%.

[0061] Preferably the discrete ceramic particles have a circularequivalent diameter in the range of about 0.1 μm to about 30 μm. Thediameter of the particles may be varied according to the desired use fora composite.

[0062] When the composite powder is to be used to coat a substratematerial (eg metal blades etc), the composite powder is thermally orplasma sprayed onto a substrate material (eg on engine component) attemperatures above about 1000° C. The techniques of thermal or plasmaspraying (or alternatives) will be known in the art. This effectivelyforms the titanium based matrix of the composite material on thesubstrate surface that is then subject to oxidation to form a surfaceoxide layer on that base matrix.

[0063] When producing pre-selected shaped products from the compositematerial, the composite powder produced via the mechanical milling ispressed into a mould to form a powder compact of the desired shape. Thispowder compact is then sintered at temperatures above about 700° C. forat least 30 minutes to form the base composite material which is thensubject to oxidation to form the surface oxide layer. This process canbe used to form components (rotor blades and the like) that are used athigh temperatures in oxidative environments. Example 3 uses such asintered composite for isothermal and cyclic oxidation experiments.

[0064] The surface oxide layer is formed onto the titanium based matrix(plus discrete particles) by oxidation of the surface of the matrix.When the titanium based composites are exposed to air or other oxygencontaining atmospheres at temperatures in the range of 700° C. to 900°C., or higher, for a suitable period of time, an oxide layer (or scale)forms on the outer surface of the composite exposed to the oxygencontaining environment from oxidation of the matrix base. The oxidelayer is primarily composed of TiO₂ but may also include some Al₂O₃particles. Depending on the composition of the titanium based composite,temperature and oxygen partial pressure in the atmosphere, the oxidelayer may take any time from about 1 to about 200 hours to form.

[0065] The oxide layer forms on the base matrix and around the discreteceramic particles in the base matrix and, as a result, the particlescross the boundary between the base matrix and the oxide layer,assisting in tying the two layers together.

[0066] Typically, the oxide layer formed on titanium based alloys orintermetallic compounds, such as Ti₃Al have traditionally had poor scalespallation resistance under the thermal stress generated by cyclicheating of and cooling to a temperature in the range of 600° C. to 900°C. Under such conditions, the oxide layer becomes cracked and falls off.This process is called spallation. The spallation of the outer oxidelayer exposes the under surface of the oxide layers which rapidlyoxidises and cracks. This cycle of rapid oxidation, cracking andspallation leads to the eventual degradation and destruction of thetitanium based materials.

[0067] It has been surprisingly found that the titanium basedcomposites, which include discrete particles, as are herein described,have superior high-temperature (ie above 600° C.) oxidation resistance.They also show superior resistance to spallation caused by thermalstress generated by cyclic heating and cooling; from room temperature toabout 900° C.

[0068] Not wishing to be bound by a specific hypothesis, it appears thatthe discrete particles embedded in the composite form a network in thecomposite that crosses into the oxide layer. The oxide layer forms atight adhesion with the discrete embedded particles as well as theTi(Al,O) matrix. At high temperatures, the interface between the oxidelayer, the Ti(Al,O) base matrix and the discrete particles shows no signof detachment, indicating an excellent adhesion between these phases, asshown by the double arrows in FIG. 4.

[0069] Thermal cyclic experiments have been conducted using the titaniumbased composites which include discrete particles. Ti(Al,O)/Al₂O₃,Ti(Al,O)/SiC and Ti(Al,O)/TiC composites which include discreteparticles have shown resistance to cracking of the oxide layer attemperatures as high as 900° C., thus avoiding spallation and subsequentoxidation of the composite. This is further described in Example 3.

[0070] The surprisingly good adhesion of the oxide layer to thecomposite surface in high temperature oxygen containing environments isalso believed to be due in part to the composition of the oxide layer.The oxide species which form the oxide layer of the composite surfaceappear to be able to form strong bonds with the discrete particles andthus allow the particles to serve as mechanical locking devices to aidthe adhesion of the oxide layer to the surface of the composite.

[0071] As the outer oxide layer of the titanium based composites hereindescribed are substantially resistant to cracking and spallation at hightemperatures of up to about 900° C., the composites have broad use in avariety of engineering applications.

[0072] A further aspect of this invention therefore is the use oftitanium based composites, including a titanium based matrix, an oxidelayer, and discrete ceramic particles, as a coating to provideprotection to engineering components (substrates) which will be used inhigh temperature oxygen containing environments up to about 900° C.Without being restrictive, the protective coating may be applied toengine blades; outer surface coatings on engine surfaces; or automobilebrake surfaces amongst others. In this way the composite may be used toprovide protection to the substrate surface against an oxygen containingatmosphere. As the composite is stable at temperatures as high as 900°C., the likelihood of the composite decomposing or degrading, exposingthe base material to oxidation, is substantially reduced. This alsoreduces the likelihood of damage to an engine, for example, by thedecomposition of the composite.

[0073] Another significant advantage of the composites herein described,particularly when used as a coating for substrates such as enginecomponents (for example) is that the coated component can be pre-treatedto form the surface oxide layer (as has been described previouslyherein) before the component is put in place on the engine. Thecomponent is thus pre-prepared in a controlled manner for use in thehigh temperature environment.

[0074] Alternatively, simple use of the component in the hightemperature environment (ie above 600° C.) would cause the oxide layerto form in situ. This is less preferred as the surface oxide layerformation is less able to be controlled but the option is available.

[0075] As discussed earlier, the composite can be applied in a number ofways. In a preferred embodiment the composite can be prepared in apowder form, and the composite powder may be sprayed on to a substrateby using a thermal or plasma spray process where the powder is heated toa temperature above typically 1000° C. and then blown on to the surfaceof a component at a high speed, using the known art of thermal spray andplasma spray processes.

[0076] With reference to the Figures, the titanium based compositeTi(Al,O)/Al₂O₃, shown in FIG. 1 was produced by sintering the TiO₂/Alcomposite powder produced by high energy mechanical milling of TiO₂powder with an aluminium metal reducing agent. The mechanical millingtypically occurs at temperatures below 100° C. for 0.5 to 10 hours underargon or vacuum. The quantity and/or proportion of reactants used isgiven in Example 1. The component ratios can be varied as desired by theuser. This high energy mechanical milling process is as specified inPCT/NZ98/00124 and its corresponding U.S. Pat. No. 6,264,719, thedisclosure of which is incorporated herein by reference.

[0077] Typically, at the end of such a milling process, there will beproduced an intermediate blended powder comprising fine fragmentsincluding a fine mixture of TiO₂ and Al phases. The powder fragmentsinclude TiO₂ and Al particles with a fragment size of each phase ofabout 500 nm.

[0078] To produce products from new composite materials according tothis invention, the TiO₂/Al composite powder is then pressed into amould and sintered under an inert atmosphere (eg Argon or similar gases,or under vacuum) at a temperature in the preferred range of betweenabout 1400° C. and 1650° C. for at least 30 minutes. This produces thetitanium based composite which includes discrete Al₂O₃ particles.Preferably the heat treatment is maintained at about 1400° C. for aperiod of up to about 4 hours inclusive, although sintering temperaturesdown to about 700° C. can also be used, as discussed below. This processforms the base composite material.

[0079] In some cases unwanted reactions may occur during sintering attemperature ranges this high. When applied pressure is used the requiredsintering temperature can be lowered to a substantial extent. As anexample, where an intermediate powder to produce Ti(Al,O)/Al₂O₃composite powder is hot isostatically pressed under a pressure of about200 MPa, the sintering temperature may be reduced to about 1200° C. toachieve fully dense Ti(Al,O)/Al₂O₃ composite powder.

[0080] When forming composites with SiC or TiC for example, to achieve afully dense Ti(Al,O)/SiC or Ti(Al,O)/TiC composite powder compact, atemperature of 800° C. is sufficient where the intermediate powder ishot isostatically pressed under a pressure of 200 MPa. It should againbe noted that the thermal treatment during the mechanical milling stageis not used for SiC or TiC.

[0081] In the case of forming a coating of Ti(Al,O)/Al₂O₃ on a substratematerial, following thermal treatment of the composite powder (as perPCT/NZ98/00124) the composite powder is then applied to the substratemetal (eg engine component etc) as a coating, preferably via thermal orplasma spray processes. Via such application processes the compositepowder is heated to a temperature of about 1000° C. then blown onto thecomponent surface at high speed. Such application processes andalternatives to them will be known to the skilled person.

[0082] The titanium composite powder coated component, or the titaniumcomposite formed component, is then heated to a temperature of 600° C.to 900° C. in an oxygen containing environment to produce the surfaceoxide layer on the composite powder coating. The controlled oxidationenvironment can be produced using an oven or flame furnace or likeapparatus. Alternatively the component can simply be used and an oxidelayer will form in situ, under temperature/oxidation conditions.

[0083] As previously mentioned the size of the discrete particles can bevaried by modifying the composite preparation conditions, or the heatingconditions. For example, the Al₂O₃ particle size can be increased byincreasing the heat treatment temperature. If the temperature isincreased from 1500° C. to 1650° C., the Al₂O₃ particle size may beincreased on average from 15 μm to 30 μm.

[0084]FIG. 1 illustrates a SEM micrograph of a titanium based compositeof Ti(Al,O)/Al₂O₃ formed by pressureless sintering of the compositepowder compact at 1550° C. for 1 hour. The process is best described byExample 1. The Ti(Al,O)/Al₂O₃ composite is consistent with Al₂O₃ ceramicparticles embedded in the Ti(Al,O) matrix. The dark patches shown by themicrograph are Al₂O₃ particles. The circular equivalent diameter of theAl₂O₃ particles embedded in the Ti(Al,O) matrix is in the range of about0.1 μm to about 30 μm. FIG. 1 does not show the presence of the surfaceoxide layer of the composite.

[0085] With reference to FIG. 2(a), an SEM micrograph of theTi(Al,O)/Al₂O₃ composite produced by pressureless sintering of theAl/TiO₂ composite powder at 1550° C. is illustrated. The Al₂O₃ particlesare shown as dark regions in the Ti(Al,O) matrix. FIG. 2(b) illustratesa Ti(Al,O)/Al₂O₃ composite produced by pressureless sintering of theAl/TiO₂ composite powder at 1650° C. The process for production isotherwise as described for FIG. 1. The Al₂O₃ particle can be seen to besubstantially larger when the Ti(Al,O)/Al₂O₃ composite is prepared at asintering temperature of 1650° C. (FIG. 2(b)) than when theTi(Al,O)/Al₂O₃ composite is prepared at the lower temperature of 1550°C. (FIG. 2(a)). As can be seen in FIG. 2(b) more than 50% of the Al₂O₃ceramic particles are interconnected.

[0086] FIGS. 3(a) and 3(b) illustrate an SEM micrograph of a titaniumbased composite which includes SiC particles. The composites wereproduced by HiPping at 1000° C. for 2 hours under 200 Mpa. The Sipowder, Ti powder and Al powder were mechanically milled for (3 a) 2hours and (3 b) 8 hours prior to pressing and sintering. TheTi(Al,O)/SiC composites illustrated in FIGS. 3(a) and 3(b) areconsistent with SiC particles embedded in the Ti(Al,O) matrix. The SiCceramic particles are illustrated as dark regions in the micrograph. SiCparticles are in the range of 0.1 μm to 10 μm. The Ti(Al,O)/SiCcomposite is reinforced with a 10% volume fraction of SiC particles.Again, the Figures do not show the presence of the surface oxide layer.

[0087]FIGS. 4 and 5 show SEM micrographs including the oxide layerformed on the titanium based composite products formed as for FIG. 1.

[0088] FIGS. 4(a) to 4(c) show the morphologies of a Ti(Al,O)/Al₂O₃composite after exposure to an oxygen environment at a temperature of700° C. for 100 hours. The cross-over of the discrete Al₂O₃ particlescan be seen. FIG. 4(a) illustrates a Ti(Al,O)/Al₂O₃ composite in whichthe outer surface of the composite or surface exposed to air ispartially covered by the oxide layer as shown by light patches on thesurface. From the cross-section morphologies in FIGS. 4(b) and 4(c) itcan be seen that the Al₂O₃ particles are well embedded in the Ti(Al,O)matrix and are integrally associated with the oxide layer formedfollowing oxidation of the matrix.

[0089] FIGS. 5(a) and 5(b) illustrate a cross section of a titaniumbased composite which includes TiC ceramic particles (Ti(Al,O)/TiC). Thecomposite was produced as for the SiC material of FIG. 3. FIG. 5a showsa product produced following 8 hours mechanical milling and 5 b after 16hours milling. The Ti(Al,O)/TiC composite compact sample was heated to800° C. in air for 200 hours. The TiC embedded particles are not visiblein the SEM micrographs. The oxide layer can be seen on the surface ofthe composite and is indicated.

[0090] FIGS. 6(a) to (d) illustrate SEM micrographs of the Ti(Al,O)/SiCcomposite powder produced after different milling durations. Thepresence of the SiC particles can clearly be seen as darker areas.

[0091] The invention will now be described with reference to theExamples below.

EXAMPLE 1—Preparation

[0092] The Ti(Al,O)/Al₂O₃ composite powder was fabricated through highenergy ball milling of a mixture of TiO₂ and Al powders, followed bypressureless sintering at 1550° C. for 1 to 5 hours under argon. Itconsists of ˜50 vol. % α-Al₂O₃ particle. The milling was as described inPCT/NZ98/00124. The sintered bar was cut to rectangular samples with adimension of ˜14×9×1 mm. FIG. 1 best illustrates the Ti(Al,O)/Al₂O₃produced by this process. Before oxidation test, all surfaces wereground to 1200 grit SiC, cleaned with alcohol and acetone, and thendrying in hot air.

EXAMPLE 2 Characterisation of Ti(Al,O)/Al₂O₃ Composite

[0093] Surfaces of a Ti(Al,O)/Al₂O₃ composite sample as produced inExample 1 were ground and polished down to 1 μm diamond paste for SEMobservation. FIG. 1 shows the typical micrographs. The dark patches areα-Al₂O₃, while the white patches are Ti(Al,O) matrix. In the macroscopicscale, α-Al₂O₃ particles incorporated into Ti(Al,O) homogeneously.However, it could be observed that smaller particles of α-Al₂O₃ andTi(Al,O) were mixed with each other.

EXAMPLE 3 Isothermal and Cyclic Oxidation

[0094] Isothermal oxidation tests on the product produced by Example 1were carried out in a horizontal tube-furnace. After a designed timeperiod of exposure, the specimens were pulled out of the hot zone,cooled in a desiccator, then weighed with an electronic balance with anaccuracy of 0.01 mg. Cyclic oxidation was performed in a verticalfurnace. Oxidation in furnace and cooling in air periods were 60 min and10 min, respectively. Under both testing conditions, the specimens wereheld in quartz crucibles. Therefore, scale spallation could be clearlyobserved and measured.

[0095] The isothermal and cyclic oxidation kinetics of theTi(Al,O)/Al₂O₃ composite sample in air at 700° C., 800° C. and 900° C.were studied. In general, the kinetic behaviours for isothermal andcyclic tests were very similar for Ti(Al,O)/Al₂O₃, and could be fittedwell with parabolic rate law, showing that the diffusion of reactants isthe rate-controlling process during oxidation. The apparent oxidationrates were reasonably low, much lower than those of unalloyed Ti₃Alintermetallic compounds, but slightly higher than those of Ti₃Al-11 at.%Nb. The mass gains and parabolic rate constants measured are shown inTable 1. TABLE 1 Data on Mass Gain of Ti(Al,O)/Al₂O₃ composite andTi₃Al-based Alloys Temperature/ Time/ Alloy Environment ° C. hours MassGain/mg/cm² Ti-21 at. % Al Air 700 50 0.4 (I) Ti-24Al-11Nb 100 0.36 (I)Ti(Al,O)/Al₂O₃ 100 0.46 (I)/0.61 (C) Ti-25Al Air 800 100 8.45 (I)Ti-24Al-11Nb 24 0.36 (I) Ti₃Al-Nb 100 2.07 (I) Ti(Al,O)/Al₂O₃ 30 1.38(I)/1.40 (C) Ti(Al,O)/Al₂O₃ 100 2.33 (I)/2.09 (C) Ti-25Al Air 900 100108.5 (I) Ti-24Al-11Nb 24 1.18 (I) Ti-24Al-11Nb 100 3.86 (I)Ti(Al,O)/Al₂O₃ 30 4.31 (I)/3.53 (C) Ti(Al,O)/Al₂O₃ 100 8.12 (I)/6.38 (C)

[0096] An important feature of the Ti(Al,O)/Al₂O₃ sample was that nooxide layer or scale spallation could be observed anywhere under alltesting conditions, evidence of superior scale adherence even undersevere thermal cycling.

[0097] In correspondence with the similar oxidation kinetics, thesurface and cross-section morphologies of the heat treated samples afterisothermal and cyclic oxidation were also basically the same.

[0098] At 700° C., the oxide formed was a mixture of TiO₂ and Al₂O₃ inthe form of small grains and whiskers. The oxide layer covered almostone half of the original surface area (i.e. the oxide layer was onlyformed on the top of the original Ti(Al,O) matrix (FIG. 4(a)). A verythin Al₂O₃ layer could be identified near the outer surface of the oxidelayer, while the main body was composed of relatively dense TiO₂ with asmall amount of Al₂O₃. The rugged reaction front penetrated into thesubstrate with a depth of about 10 μm. The interface between oxide layernewly formed and original α-Al₂O₃, and the interface between oxides andTi(Al,O) matrix showed no sign of detachment, indicating excellentadhesion between these phases.

[0099] At 800° C., the Ti(Al,O)/Al₂O₃ surface was covered by largeclusters of oxides forming the oxide layer. In comparison with thesurfaces oxidised at 700° C., these clusters spread to cover moresurface area. It could be seen that these clusters were composed ofrelatively small and dense oxide particles. EDS analysis showed thatthese were mainly rutile crystals with a small amount of α-Al₂O₃. Alayer of Al₂O₃ formed underneath the surficial TiO₂ oxide layer. Theoriginal α-Al₂O₃ particles in the composite were still dense, showing nosigns of cracking or spallation, and good adherence to the oxideparticles newly formed and the unoxidised Ti(Al,O) matrix.

[0100] At 900° C., the oxide layer of Ti(Al,O)/Al₂O₃ specimens afterisothermal and cyclic oxidation was covered with rutile crystals, whichshowed coarse size and random orientation. A multi-layeredmicrostructure could be observed on the isothermal oxidised specimen.The outer face of the oxide layer is a thick rutile layer, under whichis a nearly continuous alumina layer, then thick TiO₂ with a smallamount of α-Al₂O₃. Porous layer of 5-10 μm was observed at the interfaceof oxide layer composite after isothermal oxidation. On the specimensafter cyclic oxidation, however, very few pores could be seen at thisinterface. The penetration of the reaction front was similar for bothisothermal and cyclic oxidation.

EXAMPLE 4 Oxidation Kinetics

[0101] In general, oxidation of the Ti(Al,O)/Al₂O₃ composite exhibitsmuch lower rates in comparison with Ti₃Al intermetallic composite. Asstated, isothermal and cyclic oxidation behaviours are very similar.Thermal stresses generated during cyclic oxidation do not result inoxide scale detachment or spallation. The oxide scales formed under bothconditions have superior adhesion and spallation resistance.

[0102] As shown in Table 1 (above), the mass gains and the parabolicrate constants of Ti(Al,O)/Al₂O₃ composite are much lower than those ofTi₃Al intermetallic composite, but slightly larger than those ofTi₃Al-11 at. % Nb alloy—the best oxidation resistant alloy in Ti₃Al—Nbsystem.

[0103] The low oxidation rates of the Ti(Al,O)/Al₂O₃ composite may bepartially attributed to the ˜50 vol. % α-Al₂O₃ phase in the composite,which does not oxidise further. The inward diffusion and incorporationof oxygen through a reduced surface area, results in a low apparentoxidation rate. However, this does not completely account for the ˜10fold reduction in the oxidation mass gains.

[0104] The foregoing describes the invention including preferred formsthereof. Modification or alterations as would be apparent to a skilledperson are intended to be included within the scope and spirit of theinvention as defined in the attached claims.

What we claim is:
 1. A titanium based composite which includes aTi(Al,O) base matrix, discrete ceramic particles, and an oxide layer onthe surface of the composite, wherein the discrete ceramic particles areintegrally associated with the Ti(Al,O) base matrix and the oxide layer,and wherein, at a temperature of above about 600° C., the composite issubstantially resistant to oxidation and/or spallation.
 2. The compositeaccording to claim 1 wherein the discrete ceramic particles range insize from about 0.1 μm to about 30 μm.
 3. The composite according toclaim 1 wherein the discrete ceramic particles are selected from Al₂O₃,SiC, TiC, TiN, TiB₂, Y₂O₃ and/or Si₃N₄.
 4. The composite according toclaim 1 wherein the discrete ceramic particles may constitute a volumefraction of about 10% to about 60% of the titanium based composite.
 5. Acoating material including titanium based composite adapted for use onsubstrate components used at high temperature and/or in oxidativeenvironments, wherein the composite includes a Ti(Al,O) base matrix,discrete ceramic particles and an oxide layer, wherein the discreteceramic particles are integrally associated with the Ti(Al,O) basematrix and the oxide layer so that at a temperature of above about 600°C. the composite is substantially resistant to oxidation and/orspallation.
 6. The coating material according to claim 5 wherein thediscrete ceramic particles range in size from about 0.1 μm to about 30μm.
 7. The coating material according to claim 5 wherein the discreteceramic particles are selected from Al₂O₃, SiC, TiC, TiN, TiB₂, Y₂O₃and/or Si₃N₄.
 8. The coating material according to claim 5 wherein theceramic particles constitute a volume fraction of about 10% to about 60%of the titanium based composite.
 9. The coating material according toclaim 5 wherein the composite is resistant to oxidation and/orspallation at temperatures between about 600° C. and about 900° C. andmore preferably above 700° C.
 10. A method of producing a coating forapplication to a component used at temperatures above about 600° C.and/or in oxidative environments, wherein the method includes the stepsof: preparing a Ti(Al,O) based composite powder, with each of the powderparticles including discrete Al₂O₃ particles, according to themechanical milling and thermal treatment method disclosed inPCT/NZ98/00124; applying the composite powder produced to a substratecomponent to produce a composite coating; and exposing the coatedcomponent to a high temperature, oxidative environment above about 600°C. to form a surface oxide layer on the composite coating.
 11. Themethod according to claim 10 wherein the composite powder is applied tothe substrate using a thermal or plasma spray process.
 12. The methodaccording to claim 10 wherein the coated component is heated to betweenabout 700° C. and about 900° C. for between about 1 and about 200 hoursin an oxygen containing environment to form the surface oxide layer. 13.The method according to claim 10 wherein the coated component is heatedin an oven before use or is heated in situ during use.
 14. A process forproducing a titanium based composite material in a pre-selected formincluding the steps of: preparing a Ti(Al,O) based composite powder witheach of the powder particles, including discrete Al₂O₃ particles,according to the mechanical milling and thermal treatment methoddisclosed in PCT/NZ98/00124; pressing the powder formed into apre-selected mould to produce a powder compact and sintering the powdercompact at a temperature of above about 700° C. under an inertenvironment; exposing the sintered composite material or component to ahigh temperature, oxidative environment above about 700° C. to form asurface oxide layer; wherein the product produced is substantiallyresistant to oxidation and/or spallation at temperatures above about600° C.
 15. The process according to claim 14 wherein the sinteringtemperature is between about 700° C. and about 1650° C.
 16. The processaccording to claim 14 wherein the inert environment is a vacuum or argonenvironment.
 17. A method of producing a coating for application to acomponent used at temperatures above about 600° C. and/or in oxidativeenvironments, wherein the method includes the steps of: preparing aTi(Al,O) based composite powder, with each of the powder particlesincluding discrete TiC, SiC, TiN, TiB₂, Y₂O₃ and/or Si₃N₄ particles,according to the mechanical milling method disclosed in PCT/NZ98/00124;applying the composite powder produced to a substrate component toproduce a composite coating; and exposing the coated component to a hightemperature, oxidative environment above about 600° C. to form a surfaceoxide layer on the composite coating.
 18. The method according to claim17 wherein the composite powder is applied to the substrate using athermal or plasma spray process.
 19. The method according to claim 17wherein the coated component is heated to between about 700° C. andabout 900° C. for between about 1 and 200 hours in an oxygen containingenvironment to form the surface oxide layer.
 20. The method according toclaim 17 wherein the coated component is heated in an oven before use oris heated in situ during use.
 21. The method according to claim 17wherein the component is to be used at temperatures between about 600°C. and about 900° C.
 22. A process for producing a titanium basedcomposite material in a pre-selected form including the steps of:preparing a Ti(Al,O) based composite powder, with each of the powderparticles including discrete TiC, SiC, TiN, TiB₂, Y₂O₃ and/or Si₃N₄particles, according to the mechanical milling method disclosed inPCT/NZ98/00124; pressing the powder formed into a pre-selected mould toproduce a powder compact and sintering the powder compact at atemperature of above about 700° C. under an inert environment; exposingthe sintered composite material or component to a high temperature,oxidative environment above about 700° C. to form a surface oxide layer;wherein the product produced is substantially resistant to oxidationand/or spallation at temperatures above 600° C., preferably betweenabout 600° C. and about 900° C.
 23. The method according to claim 22wherein the sintering temperature is between about 700° C. and about1650° C.
 24. The method according to claim 22 wherein the inertenvironment is a vacuum or argon environment.
 25. A product produced ina pre-selected form when produced by the process of claim 14 or claim22.
 26. A component including a coating produced according to the methodof claim 10 or claim 17.